The present invention relates to a porous composite structure, in particular a membrane for separating fluids, which comprises at least a porous support and a zeolite layer applied to the support and in which the zeolite is a zeolite of the T type or of an erionite type.
In recent years the separation of organic and aqueous phases has formed an important area in the development and use of membranes and membrane processes. One important field of application is for example the separation of azeotropic mixtures or mixtures having a narrow boiling point range by means of pervaporation or vapour permeation. In this process the mixture to be separated (the feed) is applied to the membrane in the form of a liquid (pervaporation) or a vapour (vapour permeation). The mixture is separated into two streams via the membrane: into a permeate which is separated off via the membrane and has a considerably higher concentration of water than the feed stream, and a retentate, the water content of which is lower than that of the starting mixture.
The currently most widely developed membranes for the above applications are organic dense polymer membranes, such as for example polyvinyl alcohol membranes, as described in U.S. Pat. No. 2,953,502, which are used for the separation of azeotropic alcohol/water mixtures. The selectivity of these organic membranes is limited. It is for example highly complicated to separate a methanol/water mixture by means of organic membranes and no advantages over distillation are therefore provided. In addition, organic membranes do not have sufficient thermal and chemical stability. The characteristic temperature resistance of up to a maximum of 100xc2x0 C. and the limited solvent resistance (such as for example to DMF or acetonitrile) considerably restrict the fields of application of organic membranes.
More recent developments have been focussed on inorganic membranes which provide comparable or higher selectivity than dense organic membranes. U.S. Pat. Nos. 5,258,339 and 4,699,892 describe the production of a composite membrane comprising a separating zeolite layer not described in more detail and a porous inorganic supporting layer. Offenlegungsschrift EP 0,659,469 provides a more detailed description of the structure of a membrane comprising a separating zeolite layer of the NaA type and a porous support for separating liquid mixtures, such as for example alcohol/water mixtures.
The use of these membranes for separating acidic organic/aqueous mixtures of the kind frequently encountered in industry, in particular in reaction processes, is not possible due to the pH instability of the NaA zeolite layer applied. When in contact with aqueous solutions of a low pH value the zeolite layers decompose within a very short time, i.e. the active layer of the membrane is destroyed, so that the selective separation of water from acidic organic/aqueous mixtures is not possible using such membranes. The same applies to a zeolite layer of the NaY type, as described in JP 08257301.
In laid-open specification JP 08257302, a resistant inorganic zeolite membrane is described in which the separating layer comprises a ZSM5-type zeolite. Although this zeolite is resistant to acids, it is hydrophobic and thus predominantly separates hydrophobic substances. It is not particularly permeable to water. A membrane having a layer of a ZSM-5 zeolite is therefore not suitable for removing water from aqueous/organic systems.
FR 2 719 238 describes a different structure for a composite membrane comprising zeolites and an inorganic support. The zeolite only fills the large pores of the support material and separation therefore takes place via the zeolite crystals located within the large pores of the support material. The production of such a structure in a defect-free, i.e. dense form, is difficult and is only possible if the zeolite penetrates deeply into the pores of the support material. The resulting large zeolite thicknesses greatly impede mass transfer and the permeation flow rates are therefore low. The use of such membranes is thus comparatively ineffective.
The problem on which the invention is based is that of providing a membrane which does not have the disadvantages of known membranes, is suitable for separating organic/aqueous mixtures, in particular acidic phases, provides high selectivity and sufficient permeation flux and has a long service life given sufficient temperature stability.
The above problem is solved according to the invention by a porous composite structure, in particular a membrane, which comprises a porous support structure to which a film consisting of a zeolite of the T type or of an erionite type is attached, has high selectivity and a high permeation flow rate as well as high acid stability and which is suitable for separating acidic, organic/aqueous mixtures of the kind frequently encountered in industry, in particular in conjunction with chemical reactions, by means of pervaporation, vapour permeation and gas permeation.
The present invention relates to a porous composite structure, in particular a membrane for separating fluids, which comprises at least a porous support and a zeolite layer applied to the support and in which the zeolite is a zeolite of the T type or of an erionite type.
The composite structure according to the invention comprises a porous support material and a zeolite applied thereto which forms a defect-free, dense layer. The zeolite applied is a zeolite of the T type or of the erionite type. This zeolite has high stability towards acids and organic solvents. At the same time, by virtue of its hydrophilic properties and its small pores in the form of eight-membered rings (the pore diameter typically being 3.6xc3x975.1 xc3x85) it allows the selective removal of water from mixtures. This composite structure avoids the abovementioned disadvantages of known membranes and is excellently suitable in particular for the separation of water from acidic organic/aqueous mixtures.
It has been found that in particular a zeolite of the T type or erionite type displays the above properties and is therefore especially suitable for the production of the separating layer. The composite structure can be produced in a hydrothermal process in which the zeolite layer is crystallized directly onto the support at low temperatures.
The term xe2x80x9cerionitexe2x80x9d refers to the naturally occurring variant of the abovementioned zeolite. A comparable synthetically produced zeolite is referred to as T-type zeolite, which is always a mixture comprising erionite zeolite (in a molar proportion of 0.5-0.95) and offretite zeolite (in a molar proportion of 0.5-0.05).
The suitable porous support can consist of a ceramic material or a metal oxide, such as for example aluminium oxide, silicon dioxide, zirconium oxide, silicon nitride, silicon carbide etc. or of a metal, such as for example aluminium, silver or special steel or of organic polymers, polypropylene, polyethylene, polytetrafluoroethylene, polysulphone and polyimide.
The support preferably has an average pore diameter of 0.05 xcexcm-10 xcexcm, in particular 0.1 xcexcm-2 xcexcm, and a porosity of 10% to 60%, preferably 30% to 50%. The porosity or degree of porosity is understood to be the ratio of the pore volume to the total volume of the support structure. Smaller pore diameters than 0.05 xcexcm are not suitable due to the insufficient permeation flow rates. A porosity of less than 10% also produces a large reduction in the permeation flow rate. If the pore diameter is larger than 10 xcexcm a decrease in selectivity may occur. A porosity of higher than 60% also results in a decrease in selectivity and in the strength of the material.
A particularly preferred support for the composite structure comprises aluminium oxide with an average diameter of 0.1 xcexcm-0.2 xcexcm and a porosity of 30%-50%, and contains 50% to 100% Al2O3.
The porous support is not subject to any limitations from the point of view of its external geometry. An advantageous geometry for pervaporation and vapour permeation consists of tubes of a length of 10-100 cm and having an external diameter of at least 10 mm and a tube thickness of at least 0.2 mm to several millimetres. The zeolite layer can be applied to the internal and/or external surface of the tubular support structure, and preferably to the external surface. The porous structure can also be a cylindrical structure having an external diameter of 30-100 mm and a length of 20-100 cm and a large number of longitudinal channels with diameters of 2-12 mm.
A T-type zeolite or an erionite zeolite is synthesized on this support material in a hydrothermal process in which silicon dioxide powder or sodium silicate are used as the silicon sources, sodium aluminate and aluminium hydroxide as the aluminium sources and sodium and potassium hydroxide as the cation sources. A specific mixture comprising the abovementioned starting substances is suspended in distilled water, stirred and allowed to age for a specific time (1-48 hours). Then the support is immersed in this solution and the zeolite layer is applied by hydrothermal synthesis under specific conditions. The porous support preferably contains seed crystals of an average size of less than 50 xcexcm, and preferably zeolite crystals of an average size in the range from 10-150 xcexcm. The quantity of seed crystals on the porous support is 1-500 mg/cm2, preferably 10-60 mg/cm2.
In addition to hydrothermal processes, other processes, such as for example gas phase deposition, can be used for the coating of the porous support. The hydrothermal process for the production of the zeolite layer is carried out at a temperature in the range from 60 to 1 50xc2x0 C., preferably 80-110xc2x0 C., and for a period of 1 to 48 hours, preferably 20-24 hours, followed by washing for a period of 6-24 hours and then drying again.
A particularly preferred composite structure is characterized in that the zeolite film of the T type or erionite type forms a defect- and crack-free layer on the support material.
The zeolite film of the T type or erionite type is formed on the composite structure particularly by hydrothermal deposition using starting materials in the following molar ratios:
The zeolite film of the T type or erionite type preferably has a layer thickness of from 3 to 100 xcexcm, and more preferably from 10 to 50 xcexcm.
If the zeolite film of the T type or erionite type is formed on both sides of an in particular tubular, porous support, the thickness of the zeolite film is in particular 10-50 xcexcm and the total thickness of the composite structure is from 1 to 3 mm, including the support material.
The invention also relates to the use of a composite structure according to the invention for separating fluids, in particular fluids comprising organic compounds and water or organic mixtures optionally containing water, by means of pervaporation, vapour permeation or gas permeation.
The composite structure according to the invention is particularly preferably used for separating water from acidic (pH value greater than 7, preferably greater than 4) organic/aqueous mixtures by means of pervaporation, vapour permeation or gas permeation.
The membrane according to the invention can be used for the selective separation of water from liquid or vaporous mixtures comprising alcohols (such as for example methanol, ethanol, propanol, etc.), ketones (such as for example acetone and methyl ethyl ketone, etc), halogenated hydrocarbons (such as for example carbon tetrachloride and trichloroethylene, etc.), gases, such as for example CO2 and N2, or generally aqueous, acidic mixtures which may contain for example hydrochloric acid, acetic acid or phenol or which comprise two or more of the above components, by means of pervaporation and vapour permeation.
The membrane according to the invention having a coating of a zeolite of the T type or erionite type differs from other currently available hydrophilic membranes in its capacity to selectively separate water from organic/aqueous mixtures, in particular from mixtures containing acidic components, of the kind frequently encountered in industry, at high permeation flow rates, and in its long-term stability in acidic media.
Such membranes can therefore be used in membrane reactors in the presence of acidic components for the systematic removal of a particular reaction product. They represent energy- and space-saving coupling devices in reaction and membrane technology.
The invention is illustrated in more detail below, with the aid of FIGS. 1 to 5, without being specifically limited thereby.
FIG. 1 depicts the schematic layout of a unit for separating ethanol and water using a tubular composite structure according to the invention.
FIG. 2 depicts diffraction intensity curves of three T-type membranes with different coatings and a non-coated support.
FIG. 3 depicts diffraction intensity curves of four T-type zeolite membranes with different synthesis times and a non-coated support.
FIG. 4 depicts diffraction intensity curves of T-type membranes of the same synthesis composition but a different synthesis temperature as well as a membrane produced from a non-aged solution.
FIG. 5 depicts, in relation to the separation of water from phenol at a feed temperature of 80xc2x0 C. (permeate pressure 10 and 5 mbars), the water content in the permeate and the water flow rate QH2O as a function of the water concentration in the feed.